Implantable medical sensors are used for sensing physiological signals in a patient for use in diagnosing a disease state or managing patient therapies. Among examples of implantable sensors are pressure sensors, flow sensors, acoustical sensors, and optical sensors. A pressure sensor positioned in the heart or in a blood vessel is highly useful in monitoring cardiovascular conditions, such as heart failure, hypertension, arrhythmias or other conditions. For example, a capacitive pressure sensor includes one capacitor electrode along a diaphragm and a second capacitor electrode substantially parallel to and held a few micrometers from the electrode of the diaphragm. An “air gap” provides insulation between the two parallel electrodes. As the blood pressure changes, the diaphragm flexes closer to or further away from the second electrode, resulting in a change in capacitance. The capacitance can be measured in many ways and can be converted to pressure using a calibration algorithm.
Some sensors can produce a baseline signal that drifts over time or presents a baseline offset. The causes of drift or offset can vary and lead to erroneous measurements determined from the sensor signal, particularly for calibrated sensors such as a calibrated pressure sensor. Accordingly, sensor designs are needed that reduce the potential for error in chronically implanted sensors.